Flatline Compressor With OTA

[liquids]

This is the concept I'm pondering and asking for some tips on - taking the functionality and simplicity of the flatline compressor's circuit, but eliminate the vactrol, use and OTA. 

IF you ask, "Why?"  Well, vactrols are getting more and more expensive for one...I don't want to order anymore at the molment, and I have more OTAs on hand so I'm back to giving them a go. 

The circuit that I want to implement this in is already large, so simplification of intense compression is ideal (the flatline to a T).  While sorta simple and a cousin, the ross compressor somehow doesn't sustain as intensely as the flatline and EHX (DOM, Synth, Attack/Decay) compressors do.   

This part I get - put the OTA in the feedback loop as a sub for the LDR.  This means attenuating the signal and feeding it to one of the OTAs input, hooking the output to the amplifying op-amps negative pin...or just rip that portion right out of the Paia hot lyx or EHX designs etc with compressors using OTAs in an op amp feedback loop.  Ok....I think.

Where I am lost: how/where do I tap the rectification portion that normally drives the LED, and use it to feed the OTA's control input...probably done ideally with a transistor from what I see?   I like the simplicity of that rectifier, with an LED it does work well, if I can make it work with an OTA just as well, I'd like to.

While the hot lyx is 'simple enough' circuit to a degree, and similar to what I'm looking for I think, assuming it can offer extreme compression like the EHX and flatline....the rectification section seems a bit thick - I don't quite get the way C18 and R17 should connect, and when I tried it (and variations on how those could connect) I confess I couldn't get it to work satisfactorily, tossed it aside prematurely since it seemed only slightly less bulky and probably inferior to the EHX circuitry.

Anyhow, any help you or anyone can offer a man beating his head against the wall (that wall being the limitations of his electronics capabilities) is appreciated.

References:
http://www.next.gr/uploads/312-b9ab16e18d.jpg
http://www.freeinfosociety.com/media/images/4113.jpg
http://wwww.paia.com/talk/download/file.php?id=77&sid=8f65689d5d34489355ef1cf4fdbd513b

[seedlings]

Granted, an LDR is $0.25, but an LED is only $.02 USD at the writing of this post. 

In other words, Why can't you employ an LED / LDR combo in place of the vactrol?

CHAD

[merlinb]

While sorta simple and a cousin, the ross compressor somehow doesn't sustain as intensely as the flatline and EHX (DOM, Synth, Attack/Decay) compressors do.   

At full tilt the Ross is a limiter, not just a compressor. That's as much sustain as you can get! Sounds more like you want to tweak the decay rather than anything else...

[liquids]

Granted, an LDR is $0.25, but an LED is only $.02 USD at the writing of this post. 

In other words, Why can't you employ an LED / LDR combo in place of the vactrol?

CHAD

When I first demoed the flatline a few years back, I found it somewhere between subtle and useless.  I used an LED/LDR combo as called for...
stand alone LDRs are slow from the specs I see, that explains it. 

I gave it a try with the VTL5C3 in there right now which is fast and was startled at the amazing squash if offers.  There IS a cheaper silonex that I may screw with with specs that indicate it's equally responsive but still...

[liquids]

While sorta simple and a cousin, the ross compressor somehow doesn't sustain as intensely as the flatline and EHX (DOM, Synth, Attack/Decay) compressors do.   

At full tilt the Ross is a limiter, not just a compressor. That's as much sustain as you can get! Sounds more like you want to tweak the decay rather than anything else...

Maybe....have you ever checked out the DOM (etc) compressor?  It's just ridiculous...and is so much more intense than the Ross - it's hard to believe until you try it.  It's probably useless as a stand alone affect, but this is more as a portion of a larger signal processor like it's use in the DOM.

[merlinb]

have you ever checked out the DOM (etc) compressor? 

Nope. Just googled it, couldn't find anything? In fact, the top hit what this thread!

[seedlings]

When I first demoed the flatline a few years back, I found it somewhere between subtle and useless.  I used an LED/LDR combo as called for...
stand alone LDRs are slow from the specs I see, that explains it. 

I gave it a try with the VTL5C3 in there right now which is fast and was startled at the amazing squash if offers.  There IS a cheaper silonex that I may screw with with specs that indicate it's equally responsive but still...

Thank you for the clarification!  I had just ordered my first batch of LDRs for messing around just before I saw this post.  It's good to know that if results aren't the greatest, there is an upgrade.

CHAD

[Seljer]

I believe DOM would refer to the EHX Deluxe Octave Multiplexer

[liquids]

Yeah, the compressor is the first portion of the Deluxe Octave Multiplexer schematic, which I linked to in my first post, but the same compressor is found in the ehx attack/decay and ehx guitar synth...the best drawing around is on that 'other site'  

'yadayada'.org/download/file.php?id=10121&mode=view  

sub out the yadayada

[liquids]

Thank you for the clarification!  I had just ordered my first batch of LDRs for messing around just before I saw this post.  It's good to know that if results aren't the greatest, there is an upgrade.

CHAD

Happy to help.  I'd love to know if there are any LDRs that are really quite quick and responsive to a good LED, as compared to the results I get with the VTL5C3.  My knowledge/experience doesn't go far here, though I have a small pile of LDRs still in the packaging, from back when I was determined to find an LDR solution that was ideal for circuits (lightning fast response with linear, high-dark resistance is my ideal in any circuit IMO).    If someone else knows of such an LDR, and can blast a hole through my verbose generality of LDRs being at least slow compared to fastest available photocouplers, I'd love to know so I could order some of them myself!  

Plenty of photocouplers are slow to snail-like too, by the way!  The VTL5C2 I have is a sloth.  I'd just as soon use an average LDR/LED. Maybe it's damaged, but the specs on the datasheet don't tout speed from what I can read.  

In many write-ups about circuits, people just indicate using and LED+LDR or photocoupler as if 'any will do.'  I first tried optical junk with a tremulus lune and thankfully ordered/used a VTL5C3, had no issues, and thought nothing of it.   My bias is that I am as rather avoidant of parts mojo and annoyed by circuits where component choice is critical. TAOE reinforced this as a bad circuit design practice anyhow, but hey, this is audio electronics.  

Come to find from subjective expirimentation, a circuit that drives an optical component with am effect that responds to an optical component just invites component specification detail if not an exact production component with tight tolerances.  Go figure.  As a disclaimer to all I write though, I'm just a hobbyist hack in comparison to many around here, don't be mistaken...

Anyhow, as for current production optical compoents, the Vactec stuff gets chatted up the most around the pedal and audio circuitry realm - that's what I can see.  Prices correspond generally speaking.  However, from the datasheets of all the vactec and silonex parts I have come across (and from the parts I tell myself I can comprehend), the Silonex NSL-32SR3 which smallbear also sells, does seems to indicate quick responsive specs that would yield a big squash in the flatline compressor like the VTL5C3.  So if you want something close to what the VTL5C3 does, but only 1/2 the cost as far as smallbear's pricing currently goes, it may be worth the risk.  I have no personal experience with that part though, so I can't really offer a subjective comparison beyond my personal limitations in knowledge which I detail above.   I do hope to have one or two in hand at some point to try in a flatline to be more definitive, even if I abandon that circuit for my current purposes.  If those characteristics don't interest you for any such reason, you may be quite happy or even happier with what you find on hand than you would with the photocouplers I'm chatting up here, and I hope I am in the minority, as going back to your original post, the LDR/LED method is certainly cheaper across the board....    

[gritz]

Hi Matthew,
I've experimented with the Silonex NSL-32SR3 and I can vouch that it is quick. Optos are deceptively complex - the greater the current it's fed the quicker the resistance drops, but the longer it takes to recover (because it's recovering from a lower resistance). But it will achieve sub-millisecond attack times with an on resistance of a few hundred ohms and a recovery of 100milliseconds+ (longer than the specsheet states). A bit less peak current and the attack will be a little slower, but the recovery will be faster. Plenty of scope for experiment! Vactrols are also sensitive to the amount of time that they are fed led current. It will recover quicker from a few milliseconds of current than from a few tenths of a seconds worth. This is similar to how the human ear behaves to loud noises and may be why a well-tuned vactrol compressor just sounds great!

And yes, I've tried making my own vactrols too, but all the ldr's I tried were too slow in the attack and had a poor light to dark resistance ratio.

[liquids]

Hi Matthew,
I've experimented with the Silonex NSL-32SR3 and I can vouch that it is quick. [...]

Jonathan - thanks so much for sharing your knowledge and experiences, great info in there; it's appreciated.

[jonasx26]

.. with linear, high-dark resistance ..

Hey! About wanting a linear resistance/current-response from a vactrol; Some time ago I spent some time trying to acheive this.
I was designing a compressor/limiter and I couldn't acheive a true hard limiter without linearizing the vactrols resistance.
This proved to be quite difficult, especially when 'limited' to a single rail supply..

In the end I scrapped the vactrol-approach altogether. It's just easier to use an OTA.
I think the reason optical compressors allegedly are considered more "musical"-sounding is due to the non-linearities of the vactrols.
So a 'mathematically' perfect, superlinear optical compressor just might be equal to a VCA-design in terms of "musical quality".

My 50 öre (Swedish currency)

[Mustachio]

When I was building my flatline I ran across this page with lots of good info.

http://home.orange.nl/fvrijswijk/guitar_electronics_diy/flatline_compressor_john_hollis/flatline_compressor_john_hollis.htm

I ended up going with a VTL5C2 and I was happy with it. It doesn't give lots of squish but I wasn't looking for that in an optical comp was looking for more of that smooth transparent sound and I felt it achieved it nicely.

I believe my old fender deluxe 85 uses vtl5c2 for the limiter on the lead channel.

Using the VTL5c2 in this circuit worked well for me with a bass, it didn't rob it of any lows gave it a lil more meat in the mids . I felt it gave a fuller sound and nice sustain. Squish just wasn't there. I normally Play guitar with a comp on all the time I never turn them off and I prefer lots of squish I normally run a monte allums opto plus modded boss CS-3 or a Dynacomp with jangle box mods. Gonna try the orange squeezer next cause Ive read it has a lot of squish to its squash! xD

Also I believe these vactrols respond a little different once its been on for a lil while like 5-10 minutes of feeding signal into them I think the led warms up and is more responsive? something like that. heh anyway I probably didn't give much new good advice but that link was a good read when I first built mine. I do remember something about comparing T-on and T-off of the different vactrols .

http://www.datasheetcatalog.org/datasheet/perkinelmer/VT500.pdf

page 46 ^

Sorry if I didn't give any new info.